The present invention relates to an improved system and method used for processing color images from a sequentially illuminated object to form high resolution color video signals suitable for use in viewing the object on a video monitor.
It will be appreciated by those skilled in the art that the use of charge coupled devices (CCD) as sensors in video imaging systems has become quite popular, as, for example, where small size and low power consumption is desired. In the processing of color video images, it is preferred for a number of reasons that a single CCD sensor be used.
There are three basic types of solid state devices that are usable as sequential video sensors. Full frame CCD sensors employ an array of integrating CCD devices which act as photo sensitive capacitors. The images are projected onto the parallel array which acts as the image plane. The device partitions the scene information into discrete sensor elements defined by the number of pixels. The charge that accumulates in each pixel during the integration period, representing rows of scene information, is shifted in parallel fashion along rows and columns (parallel register) of the photo sensitive CCD's to an output serial register. The image data is then shifted in serial fashion to a signal sensing output amplifier during the data readout cycle. This process repeats until all rows are transferred off of the device. The output from the amplifier can then be used to re-construct the image. Because the parallel register of full frame CCD's is used both for scene detection and data readout, either a shutter or synchronized strobe illumination must be used to preserve integrity of the image.
Some full frame CCD sensors have, adjacent to the array of sensor elements, a separate but identical parallel array of non-photosensitive CCD elements for storage of image data during odd or even fields. Thus, readout of the storage CCD array can take place while the image CCD array is integrating the next image frame. These "frame transfer" detectors do not require shutters or strobed illumination. However, performance is compromised by the fact that frame transfer CCD sensors have half the number of vertical lines of resolution as the equivalent full frame device. Also, because integration is still occurring during the transfer of image data from the image array to the storage array, "smearing" of the re-constructed image can occur.
Interline CCD sensors use an array of photodiodes as integrating sensor elements. Each sensor element is electrically connected to an adjacent non-photosensitive or light-shielded CCD storage element, which are arranged in a line between each line of image sensor elements. After integration of a scene, the signal or charge collected in each sensor element or pixel is shifted all at once into the light shielded parallel CCD array. Readout from this storage CCD array then can occur during the next integration period, thus yielding continuous operation. Interline CCD sensors are manufactured in both interlaced and progressive scan formats. Interlaced sensor devices are used in conjunction with NTSC or PAL video formats. There is much more flexibility in the progressive scan devices as to integration and read out timing. Other advantages of the interline devices are that the photodiodes used as image sensor elements have a much higher sensitivity in the blue spectrum where illumination LED's are less efficient.
The advantage of a full frame or interline progressive scan CCD sensor is that they contain the complete complement of vertical lines. Therefore, higher resolution images with less "smear" can be obtained. The frame transfer and interline interlaced sensors are sometimes called video devices because they are compatible with conventional NTSC or PAL video field timing and produce interlaced output of 480 pseudo lines of resolution (every other line is temporally displaced by a field period). The advantage of the frame transfer type is that integration can proceed as read out occurs. Full frame devices give 480 total lines of true resolution but they must be shuttered or the light source strobed off during readout. Both interlaced and progressive scan interline devices, however, can be read during integration since the storage CCD arrays are not photosensitive.
Prior art sequential illumination and processing methods are deficient in either the level of resolution obtainable, in their need for a higher rate of sampling of data from the sensor, or in their relative sensitivity. For example, in U.S. Pat. No. 4,253,447 is disclosed a sequential illumination process which requires reading out of only half the field lines (corresponding to either the even or the odd fields), resulting in a 50% reduction in vertical resolution. The '447 patent also discloses a second method whereby the object is sequentially illuminated by all three primary colors during each of the odd and even field periods. Unfortunately, this method requires that the CCD data be read at three times the standard speed which is not always possible. In addition, this technique requires six field memories and for a given signal-to-noise ratio, it is less sensitive than the current invention by a factor of three.
Prior art light sources used in conjunction with a sequential color video camera include: sequentially lighting the object field with each primary color by rotating a three-color segmented filter in the path of a whim light source; sequentially illuminating the object with a plurality of solid state light emitting chips mounted in a single transparent package, as disclosed in U.S. Pat. No. 4,074,306; or sequentially illuminating the object with three white strobe lights with a different primary color filter in each of the strobe light paths, as shown in U.S. Pat. No. 4,253,447. All of these methods are troublesome.
Applicant's co-pending U.S. patent application Ser. No. 905,278, filed Jun. 26, 1992, the drawings and specification of which, as amended, are incorporated by this reference, describes a novel method of producing video images using a single sensor, sequential primary color illumination, and non-color specific RGB video data memories.
Two major problems are manifested in this prior art. The efficiencies of green and blue LED's are lower than the red LED's. The efficiency of red LED's are typically 4%, greens 0.4%, and blues 0.04%. The efficiency differences, green to red, can be solved by using multiple green LED's. However, the very low efficiency of the blue LED's make the use of an adequate number of blue LED's impractical. The prior art required that a high gain video amplifier be switched into the video signal during the blue portion of the illumination sequence to insure proper white balance. The high blue signal gain requirement causes circuit instability and limits the dynamic range of the camera.
Traditional methods of compensating for the low signal strengths have resulted in a reduced temporal or spatial resolution to the point where image quality is unsatisfactory. In addition, still images obtained from prior art video imaging systems are blurred, owing to temporal differences between the two field images and to color edge effects on moving objects.